In areas where our technology is advanced, you often get us doing better than nature because natural selection only does 'as good as necessary so as not to be outcompeted' whereas our technology can actually do 'as good as practically possible' :)
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ArmatusJun 20 '12 at 20:02

Your question could use some clarification. As stated, the premise is wrong because neither leaves nor PV cells produce energy. They can capture or store energy, they can also reflect or absorb and release energy. What pools or fluxes of energy are you interested in comparing? a common comparison is "quantum efficiency", but it is not clear if this is the comparison that you are looking for.
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DavidJun 21 '12 at 6:35

Does energy used to evaporate water count? Or the energy required to physically transport water from the soil to the air? PV cells don't evaporate or transport water. These are not accounted for in standard measures of quantum efficiency.
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DavidJun 21 '12 at 6:58

@David. Thanks, fixed the point: of course energy can not be produced/created. My curiosity indulges in the application: could we ever use plants for domestic energy requirements?. It seems photovoltaic are much better, do you agree?
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Gianpaolo RJun 21 '12 at 13:00

I don't think either could meet domestic energy requirements alone, at least not in the near future. Your question could be improved with better definitions; it still says "converting", without saying to what. See, for example, quantum efficiency and quantum yield.
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DavidJun 21 '12 at 14:03

2 Answers
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Photovoltaics made from organic sources, like Graphene, (OPVs) have the potential for efficiencies exceeding 24% in a stacked structure. A theoretical max efficiency has not been proposed, but it is expected to be less than silicon. The main benefit of OPVs is the rock-bottom price and the ability to make very tiny panels.

According to wikipedia, plants typically convert around 5% of the energy of the sunlight that hit the leaves into energy usable by the plant. Sugarcane seems to be the best, it converts up to 8% of the energy into actual biomass.

The best solar panels on the market, according to the Independent, convert 21% of energy from sunlight into usable electricity. Experimental prototypes do much better, and there are some that get over 40%. Wikipedia has a nice graph of the most efficient solar cell prototypes.

By the way, the Forbes article is only the best poly-crystalline, not the best PV in general. Poly is typically less efficient than mono. There are mono panels on the market that are better than 16%
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EnergyNumbersJun 20 '12 at 21:07

Good point! Replaced that with another link and a bigger number.
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Noah SnyderJun 20 '12 at 21:26